Well spotted Owengun, yes that is a military broad arrow on a BSA commercial barrel, something I have not seen before. RSAF Enfield were making spare woodwork with provision for the mag cutoff so that it would fit any model of SMLE, Even in 1940 BSA were using cutoffs, the elimination of the cutoff was a WW1 concession and was to be resumed once the war was over.
1940 BSA No1 MK III - Pre-dispersal? is a rifle to compare yours to. I do note the commercial BSA mark. With enfields you are always learning.